SUMMARY Glioblastoma multiforme (GBM) is the most common brain tumor in adults with current standard care offering only marginal improvement in median survival from 12 to 14.6 months. Underlying the inherent resistance to therapy and cancer recurrence is a population of brain tumor initiating cells (BTICs) with stem cell-like characteristics at the apex of tumor cell hierarchy. As a result, a novel, druggable therapeutic target of the BTICs is urgently needed to eliminate the source of resistance and recurrence of tumor. We recently made the unexpected discovery that the autism-associated gene SLC9A9 is up-regulated in a subset of GBM where it is a driver of tumor growth and migration, and associated with poor patient survival and increased aggressiveness in vivo. The gene encodes the endosomal Na+/H+ exchanger isoform 9 (NHE9) that transports protons out of the vesicle lumen in exchange for cations, to finely tune endosomal pH. The pH of the endolysosomal system is known to be critical for growth factor receptor sorting, trafficking and turnover. We showed that in BTICs, overexpression of NHE9 excessively alkalinizes the endosomal lumen, inhibiting the degradation of epidermal growth factor receptor, EGFR, conferring resistance to EGFR inhibitors (Erlotinib), and prolonging downstream signaling pathways that drive tumor growth and migration. My recent preliminary data have uncovered a previously unknown association between NHE9 and stem cell-like characteristics in GBM. The goal of this proposal is to evaluate and develop NHE9 as a novel therapeutic target to counter chemoradiation resistance to GBM therapy. In Aim 1, I will test the hypothesis that NHE9 induces reprogramming factors and stem cell-like phenotypes in GBM by increasing membrane persistence of multiple, different receptor tyrosine kinases to confer resistance to chemotherapy and radiation. In Aim 2, I will evaluate the efficacy of nanoparticles engineered to silence NHE9 expression as a novel therapeutic option in GBM. Taken together, I propose a combined mechanistic and translational approach that targets brain tumor- initiating cells in GBM. !